1. Field of the Invention
This invention relates to chemical and biological assay technology carried out in disposable plastic assemblies, and in particular the devices referred to as microfluidic systems as disclosed in U.S. Pat. No. 6,063,589, issued May 16, 2000, and co-owned and co-pending patent applications U.S. Ser. No. 08/761,063, filed Dec. 5, 1996; Ser. No. 08/768,990, filed Dec. 18, 1996; Ser. No. 08/910,726, filed Aug. 12, 1997; Ser. No. 08/995,056, filed Dec. 19, 1997; and Ser. No. 09/315,114, filed May 19, 1999, the disclosures of each of which are explicitly incorporated by reference herein.
2. Background of the Related Art
Microfluidic systems are closed interconnected networks/systems of channels and reservoirs with characteristic dimensions ranging from microns to millimeters. By introducing fluids, reagents and samples into the devices, chemical and biological assays can be carried out in an integrated and automated way. In a conventional assay, two or more fluids are mixed and incubated within a microfluidic device and during, or after, this incubation period, a reaction product may be detected. It is typically the case that this microfluidic device, specifically the depths, cross-sectional dimensions and connectivity and layout of the microfluidic systems, defines the relative volumes of these fluids.
A problem in the art is that microfluidic devices, once fabricated, do not allow the user to redefine the relative volumes of the fluids to be mixed. An additional problem in the art concerns the degree and efficiency of mixing. Because the flow within a microfluidic device is laminar, mixing is brought about through mass diffusion. A typical mixing device consists of a long capillary. Two or more fluids may enter this capillary as separate fluids and leave as a single fluid. The degree of mixing can be enhanced and the time to mix these fluids can be decreased by decreasing the cross-sectional dimension of the capillary and by increasing the length of the capillary channel, but such a device can occupy a fair amount of space within a microfluidic system.
This invention describes the use of air-ballasts and microchannels with graded surface properties to allow for bidirectional fluid flow. Bidirectional flow within microchannels allows different fluids to laminate and mix within a microchannel. The combination of air-ballasts, microchannels with graded surface properties, (passive) capillary valves and (active) wax valves allow for mixing and aliquotting of arbitrary volumes within a defined microfluidic system.